I did a couple of rounds of sanding using 150 grit, then 800 grit, then 1000 grit sand paper. When I did the 1000 I wet sanded it. After that, I rubbed old coffee grounds on it to give it a yellowish finish.
The backplate is a piece of 1x2 pine that I cut at 30degree angles on either side and sprayed it with gray Rustoleum Filler Primer. The skull is being held by a single nail and some superglue.
Monoprice Maker Select Plus
Extruder = 200C
Bed = 60C
Using white PLA 1.75mm filament.